This invention relates to a method and apparatus for reducing the irreversible capacity of a rechargeable battery, in particular lithium ion batteries, in order to increase the battery""s overall energy storage capacity.
Batteries typically exhibit irreversible capacities after the initial cycle of charging. The significant capacity lost in the first cycle results in a loss in overall battery storage capacity. The irreversible capacity is due to the formation of the solid electrolyte interface (SEI) layer in typical negative electrodes from the first cycle of charging. However, other forms of irreversible capacity may be due to additional reasons, for example, cavities in the active material of the electrode structure may need to be initially filled with lithium ions before lithium ion insertion can proceed.
The present invention is directed to a method and apparatus for reducing the irreversible capacity of a lithium ion battery by initially depositing a layer of lithium metal onto or into the electrode structure. The deposited lithium serves to form the initial SEI layer before cycling to thus reduce the amount of irreversible capacity and increase the overall battery storage capacity.
A typical electrode structure is comprised of a conducting metal substrate coated with an active material mixture. For example, a typical negative electrode consists of a copper substrate coated with a mixture of graphite and a binder such as polyvinyl di-fluoride (PVDF). In accordance with the present invention, a lithium layer is deposited onto or into the electrode active material to reduce the amount of irreversible capacity by filling voids in the active material that do not participate in the reversible lithium ion insertion process.
In accordance with a preferred embodiment, lithium metal is first deposited onto a carrier, which is then used to transfer the lithium metal to the electrode structure by the application of heat, vacuum and/or pressure. The carrier preferably comprises a long strip of plastic substrate that is preferable for a continuous transfer of lithium onto or into the electrode. In addition, this approach lends itself to commercial production. The substrate could be one of several materials such as ortho-polypropylene (OPP), Polyethylene Terephthalate (PET), polyimide, or other type of plastic. Lithium metal can be deposited onto or into one or both surfaces of the substrate. The lithium-coated plastic and the electrode material are then placed between two rollers or two plates. Lithium is transferred onto or into the electrode active material by applying heat and/or pressure in vacuum. In a preferred embodiment, the rollers or plates are heated in vacuum to about 120xc2x0 C., or within the range of 25xc2x0 C. to 350xc2x0 C. and a pressure of 50 kg/cm2 to 600 kg/cm2 is applied to the rollers or plates.
The speed of movement of the carrier electrode material through the roller pair or the plate pair is in the range of 10 cm/min. to 5 m/min. For a given speed, the length of time the materials are exposed to the heat and pressure rollers, or alternatively the heat and pressure plates, will be fixed, depending only on the lengthwise distance of the plate along the direction of the material movement. For the roller pair, deformation of the rollers results in distance in the direction of travel of the material, which adds to the actual contact time of pressure and temperature application.
The method could be used with electrodes having either single-sided coating or double-sided coating in the double-sided coating method, both sides of the metal substrate are coated with active material. The coated metal substrate is then sandwiched between two lithium-coated plastic carriers, with the lithium sides facing the active material on the coated metal substrate. All three sheets are then fed into a mechanism for applying heat and/or pressure in vacuum. As a result, lithium is transferred to both sides of the coated metal substrate.
The thickness of lithium transferred onto the electrode structure can be controlled to produce a lithium coating between about 50 Angstroms and 0.3 millimeters. Using this technology, it is expected to increase a lithium ion battery capacity by about 7% to 15%.
The above and other features and advantages of the invention will be more apparent from the following detailed description wherein:
FIG. 1 shows the electrode structure coated with active material;
FIG. 2 shows the structure of the film of lithium metal deposited on the plastic substrate;
FIG. 3A shows the roller pair system that will be used to transfer the lithium from the carrier to the electrode by applying heat and pressure in vacuum;
FIG. 3B shows the plate pair system that will be used to transfer the lithium from the carrier to the electrode by applying heat and/or pressure in a vacuum atmosphere;
FIG. 4 shows the first cycle of an example negative electrode, a SiO nano-composite electrode that has not been laminated with lithium.